The liver constitutes a central site in the absorption, binding, distribution, metabolism, excretion, and toxicogenicity (absorption, distribution, metabolism, excretion and toxicity, “ADME-T”) of xenogenous materials (i.e. materials foreign to the body in their origination). When a xenobiotic entity, such as a drug, pharmaceutical, or nutriceutical, enters a human body, it is frequently cleared (i.e., metabolically disposed of) in the liver by oxidation, reduction, hydroloysis, and/or hydration steps of biochemical reaction. Of the over a dozen different cell types that comprise the liver, the hepatocyte is the type primarily responsible for playing the role of “clearing house” or “biotransformation driver,” metabolically disposing of xenogenous material. In the liver, the hepatocyte is the cell type wherein a family of enzymes named cytochrome P-450 or CYP450 are chiefly expressed, along with other enzymes that also mediate the Phase I as well as the Phase II metabolic disposition of drugs, and other xenogenous materials. The various CYP450 isozymes collectively comprise the most important group of metabolizing enzymes that perform the role of clearing house. The field of study of how the body disposes of xenobiotic entities is frequently called Drug Metabolism and Pharmacokinetics, or DMPK. The term “pharmacokinetics” is often used in contradistinction to the term “pharmacodynamics.” Pharmacodynamics signifies the impacts and effects that a drug may biochemically exert upon a cell, an organ or an entire animal; whereas pharmacokinetics signifies the impacts, effects and ultimate disposition that a cell, organ or entire animal may biochemically exert upon a xenogenous chemical entity. In everyday language, pharmacodynamics comprises what the drug does to the body, while pharmacokinetics comprises what the body does to the drug. Toxicity, including hepatotoxicity, is a major category of pharmacodynamic effect (drug efficacy being another such major category); while drug absorption, metabolism, distribution, and excretion comprise the major categories of pharmacokinetic effects.
In addition to metabolic function, signaling interactions constitute another important category of cellular function in the liver and in other organs. Classes of proteins called chemokines or cytokines, among others, frequently effectuate signaling interactions. Modern biotechnology has led to the delineation of a variety of molecular signaling pathways in the cell. These signaling pathways not only have provided insights into the mechanisms of a cell, but also have opened opportunities to intervene with cellular processes or abnormalities. Antibodies, vaccines and other forms of chemical entities have been utilized to specifically promote, inhibit, induce, or reduce one or more signaling pathways.
Many attributes of a molecular entity must be investigated and in some cases chemically modified or improved in the course of developing that molecular entity into a therapeutically efficacious and safe compound that regulatory agencies approve for marketing and clinical use. One challenge is investigating and if necessary overcoming any toxicity that the drug candidate may directly or indirectly induce. Another set of challenges is to understand and in some circumstances to improve the pharmacokinetic properties of the molecular entity. Studying and if possible improving the efficacy of the molecular entity to achieve an intended biochemical result comprises a third set of challenges to be addressed along the path of discovering, developing, testing, and ultimately receiving marketing approval for a new drug.
To address the kinds of challenges enumerated above, in vitro cell-based assay systems are frequently utilized to simulate, measure and/or predict various functional attributes (including without limitation those elaborated in the paragraphs above) of liver cells as well as of cells from other organs comprising a mammalian organism, and of the various organs themselves. These assays may be variously utilized for analytic, therapeutic, diagnostic, or industrial purposes. However, at the current state of the art, such in vitro cell-based assay systems possess limitations that impose high costs, or that limit the simulative or predictive capacities of the assay (which in turn imposes high costs when the simulative or predictive results of the assay are found to be of no or limited use). One form of limitation that currently, frequently occurs in in vitro cell-based assay systems is that the configuration of the system causes the degree of metabolic or other functional competency of the cultured cells to remain at too low a level to yield accurate, or measurable, results that afford a useful prediction of how a chemical entity being tested on the system will interact with a cell, an organ, an organ system or an entire organism in vivo. Another challenge of cell-based systems is that the system is configured such that a level of cellular functionality, once achieved, cannot be maintained over a desirable duration of time. Another such form of limitation is that the amount of time that must be devoted to the initial incubation and/or culture of the cellular materials, prior to the time when the cells assume the higher or highest degrees of functionality of which they become capable, is a time of long duration. Improvements to the state of the relevant art, which may serve to reduce any of these or other limitations or their impacts, will provide more accurate and cost-effective means of using cellular cultures.